Underwater Detection Apparatus 100 (see FIG. 1) such as Forward Looking Sonar (FLS) systems are designed to build a two dimensional image along a vertical slice in front of the vessel as shown in FIG. 1. Underwater Detection Apparatus 100 is comprised of a plurality of Transducer Elements 1, Transmitting Beam Former 2, Receiving Beam Former 3a and 3b, Echo Display Processor 4, and Indicator 5, as shown in FIG. 2.
With such FLS systems, the depth of the seabed can be detected a distance ahead of the vessel. The FLS is mainly used as an underwater collision avoidance system to detect shallows, seawalls or possibly driftwoods well ahead of the vessel, by displaying such echoes on Indicator 5 as shown in FIG. 3.
However, the FLS systems using the split beam technique are weak at displaying echoes from multiple targets coming from different directions at the same time. This is particularly the case with the presence of a seawall ahead of the vessel.
Transducer Elements 1 of a split beam system are made of several transducer elements: 8 elements in the case of the present system.
When transmitting, the elements of Transducer Elements 1 are used as a single array. Controlling the transmitting time on each element (i.e. the relative transmitting phase on each element) controls the transmitting beam pattern of the transmitted acoustic wave in the vertical plane. The transmitting beam pattern is made so that the transmitted acoustic wave propagates ahead of the vessel from below the water surface until below the vessel, in one single ping, as shown in FIG. 4.
When receiving, the elements of Transducer Elements 1 are split into two sub arrays of elements: two sub-arrays of 4 elements in the case of the present system. A receiving beam former creates a receiving beam with each sub-array, thereby creating a pair of receiving beams, as shown in FIG. 5. This technique is called the “split beam” technique. Such echoes created by this split beam technique are displayed in Indicator 5 via Echo Display Processor 4 for processing for the display of echoes.
The split beam technique is described hereinafter.
As the width of the pair of receiving beams is thin (28.5°) compared to the transmitted beam (90°), the FLS steers the pair of receiving beams from the horizontal direction (beams steered directly ahead of the vessel) to the vertical direction (beams steered directly below the vessel). There are a total of 90 steering beams. Beams0 indicates the horizontal pair of beams, whereas beams89 indicates the pair of beams 1-degree off the vertical direction. Each pair of beams is separated by 1°. FIG. 6 shows three pairs of receiving beams, beams0, beamsφ and beams89, φ being any value between 0° and 89°, with 1° increment.
The aim of the split beam technique is to be able to determine the direction of the incoming echo. FIG. 7, shows an echo coming from a direction α relative to the acoustic axis of the pair of receiving beamsφ. In such a situation, the acoustic wave of the echo reaches sub array1 before reaching sub array2. Reference letter d the distance between centers of each sub array. The extra traveling distance of the echo to reach sub array2 is therefore d·sin α. As the arrival time of the echo on sub array1 and sub array2 is different, the phase information on sub array1 and sub array2 is different. Let's call θ the difference of phase information between sub array1 and sub array2. α and θ are linked with the following eq. 1.
                    α        =                              sin                          -              1                                ⁡                      (                                          λ                ·                θ                                            2                ⁢                                                                  ⁢                                  π                  ·                  d                                                      )                                              (                  eq          .                                          ⁢          1                )            
Where λ represents the wavelength.
Therefore, the incoming direction α can be determined knowing the phase difference θ of the incoming echo using eq. 1.
Once the 90 pairs of receiving beams are created, the following set of information is retrieved from each pair of beams:                The amplitude information of echoes.        The phase difference information between sub array1 and sub array2.        The time of arrival of echoes relative to the transmitting time.        
Knowing the 3 sets of information above, echoes of targets are plotted on the display unit of the FLS as shown in FIG. 3.
Eq. 1 shows a unique relation between α and θ. However, since the transmission beam is very broad, there are situations where several echoes from different directions are coming back at the same time. This is particularly the case with underwater structures such as seawalls, as the echoes from the seabed and the seawall are coming back at the same time, as shown in FIG. 8. A dotted line in FIG. 8 shows the equidistant positions from Transducer Elements 1.
In this situation too, as the relation between α and θ is unique, the resulting incoming direction α of these multiple echoes has to be unique.
When considered separately, these two echoes would be received for example with a phase difference and amplitude information represented by the two vectors in FIG. 9, where the size of the vector is representative of the amplitude information and the angle of the vector is representative of the phase difference information.
As both echoes are coming back at the same time, information vectors are combined and the combined vector ends up in an intermediate position, in between the two original vectors, as shown in FIG. 10.
Basically, the combined vector is pulled towards the vector with the strongest amplitude.
Therefore, when two echoes from two different directions reach the FLS at the same time, the FLS displays the resulting echo in an intermediate position between the two real target positions, as shown in FIG. 11.
As a result, seawall tends to not be displayed as extending to the water surface. FIG. 12 shows a typical FLS view taken in front of a seawall. There is a gap of several meters between the water surface and the displayed seawall.
Therefore, depending on the user and the type of vessel used, some users may wrongly think that they can proceed ahead without danger, which could lead to very dangerous situations.
As such views could lead to very dangerous situations, there is a need to be able to detect seawalls reliably and inform the user if such danger is detected. This is the purpose of this invention.
In order to do that, this invention performs a seawall detection using the fact that the amplitude information of echoes received on a receiving beam directed towards the seawall decreases with time as no echoes are coming back from behind the seawall. However, echoes of air bubbles close to the sea surface can have a similar characteristic. Air bubble echoes are mainly created by propellers of motorboats and can therefore be found a lot in places such as along the coastline and near harbors. Air bubble echoes are a very good ultrasound reflector. Therefore ultrasound waves hardly go through thick mass of air bubbles and therefore there are very few or no echoes coming back from behind such mass of air bubbles. As a result, an attenuation of the amplitude of echoes after the mass of air bubbles can happen in a similar way as after a seawall.
As characteristics of seawalls and air bubbles can be very similar, there is a need to classify echoes received with an attenuation of amplitude into either seawall echoes or air bubble echoes in order to avoid alarming the user because of a misdetection of air bubble echoes: this is the aim of this invention.